In recent years, liquid crystal display devices and organic EL (electroluminescence) display devices are in wide use in televisions and monitors of personal computers and the like. Conventionally, sample-and-hold type display devices such as liquid crystal display devices or organic EL display devices have a problem of so-called motion blur, a phenomenon where, when a moving image is displayed, a boundary between portions having different display luminance is perceived as blurry. This is a phenomenon unique to the sample-and-hold type display devices, which occurs because the display data written the last time is held until the next display data is written.
One method of preventing the motion blur is a so-called double-speed driving technology, where one frame of an inputted image signal is divided into a plurality of sub-frames to mimic the impulse display. In this technology, a signal divided into multiple sub-frames is written on a pixel in multiple times. Here, the motion blur can be suppressed by reducing the luminance of at least one of the sub-frames to a low level (close to a black display).
Besides the motion blur, liquid crystal displays also have a problem of a slow response of liquid crystal elements. That is, in the liquid crystal display device, when the input gradation level changes over consecutive frames, the luminance response level after the change may not reach the input gradation level. A technology used to compensate for the slow response time of the liquid crystal elements is an overshoot driving.
With the overshoot driving, in response to the rise or fall of the inputted gradation change, a voltage that is higher or lower than the voltage actually applied for the gradation level is applied on the liquid crystal elements to fast drive the liquid crystal elements in a forced manner. A technology where the double-speed driving and overshoot driving are combined is disclosed in Patent Document 1, for example.
FIG. 5 is a block diagram showing the configuration of the image display device of Patent Document 1. In the image display device 100 of Patent Document 1, once an image signal, where the signal of each frame period is represented by a gradation level, is inputted to a memory controller 12, the inputted image signal is divided into sub-frames according to the timing signal generated by a timing controller 11. Next, a first gradation level conversion unit 13 detects a location of a luminance change in consecutive frames of the inputted image signal, and performs an overshoot driving to improve the response time in the frame immediately after the detected luminance change. Then, based on the signal whose gradation level is corrected by the overshoot driving, the second gradation level conversion unit and the third gradation level conversion unit convert the gradation levels in the sub-frames and output them to the image display panel 20.